1. Field of the Invention
The present invention relates to zinc-bromine batteries. More particularly, the present invention relates to zinc-bromine batteries having a non-flowing electrolyte.
2. Description of the Prior Art
Zinc-bromine batteries are known in the art. Early zinc-bromine batteries included a plurality of electrodes disposed in a non-flowing, zinc-bromide aqueous solution, such as the battery disclosed in U.S. Pat. No. 3,806,368, issued to Maricle et al.
However, over time zinc-bromine batteries were developed as flowing electrolyte batteries. In such batteries, the electrolyte is circulated through a stack of electrochemical cells during charging and discharging, and is stored in external reservoirs. The main advantages of such batteries are that they have greater energy storage capacity than non-flowing batteries, the circulation of the electrolyte results in uniform zinc plating and improved thermal management of the system, and they have reduced self discharge. The term self discharge refers to the internal energy loss of a battery which occurs when a charged battery is stored. Even though a battery is not electrically coupled to a load, over time a battery will undergo a gradual loss of energy, and thus undergo self discharge. The term may also refer to the internal energy loss that occurs while a battery is discharged.
Flowing electrolyte, zinc-bromine batteries have an aqueous solution of zinc-bromide and quaternary ammonium salts, for example, methylethylpyrrolidinium bromide, with optional supporting salts, such as NH.sub.4 Cl, which is circulated through the individual cells from external reservoirs. Each cell has two portions separated by a separator, one half of the cell contains an anolyte and the other half of the cell contains a catholyte. The anolyte flows through a common anolyte manifold to each anodic half cell and the catholyte flows through a parallel common catholyte manifold to each cathodic half-cell. In the anodic half cell, one surface of a bipolar electrode acts as an anode. In the cathodic half cell, one surface of another bipolar electrode acts as a cathode. The alternating separators and electrodes are sealed together in a manner which prevents communication between the anolyte and catholyte systems.
In the discharged state, the anolyte is substantially chemically identical to the catholyte. During the process of collecting a charge, the following chemical reaction takes place:
Zn.sup.++ +2 e.sup.- .fwdarw.Zn PA1 2 Br.sup.- .fwdarw.Br.sub.2 +2 e.sup.- PA1 Br.sub.2 +2 e.sup.- .fwdarw.2 Br.sup.- PA1 Zn.fwdarw.Zn.sup.++ +2 e.sup.-
Zinc is plated on the anode, and bromine is produced at the cathode. The bromine is immediately complexed by the quaternary ammonium ions in the electrolyte to form a dense second phase which is subsequently removed from the battery stack with the flowing electrolyte. Complexation of the bromine improves the safety of a zinc-bromine battery. As should be understood, elemental bromine is highly reactive. The dense, oily fluid, or second phase, formed by complexation has a lower vapor pressure and is less reactive than elemental bromine. When the battery is charged, zinc is stored on one side of each electrode and the complexed bromine is stored in the catholyte reservoir.
During the electrical discharge process, the following chemical reaction takes place.
In this reaction, zinc is oxidized, and the released electrons pass through the bipolar electrode where they combine with molecular bromine to form bromide ions. Further, the positively charged zinc ions travel through the separator and remain in solution, and at the same time, bromide ions pass through the separator in the opposite direction and remain in solution.
While flowing electrolyte batteries have certain advantages over non-flowing electrolyte batteries, they do have certain drawbacks. In particular, flowing electrolyte batteries are generally complex, and must include external pumps, reservoirs, and appropriate manifolds, by and through which the electrolyte is circulated.
While advanced zinc-bromine batteries have been developed as flowing electrolyte systems, if an improved non-flowing electrolyte battery could be developed, it would greatly simplify the construction of zinc-bromine batteries, and this benefit could, in certain applications, outweigh the drawback of reduced energy capacity of non-flowing electrolyte batteries.
Indeed, a non-flowing design would simplify a battery by eliminating the need for pumps and reservoirs. In addition, the overall weight of the battery would be reduced.